Carbon nanotubes are nanometer-scale sized tube-shaped molecules having the structure of a graphite molecule rolled into a rube. A nanotube can be single-walled or multi-walled, dependent upon conditions of preparation. Carbon nanotubes typically are electrically conductive and mechanically strong and stiff along their length. Nanotubes typically also have a relatively high aspect ratio (length/diameter ratio). Due to these properties, the use of nanotubes as reinforcements in composite materials for both structural and functional applications would be advantageous.
It is well-known in the field of composites that the reinforcement fiber orientation plays an important role in governing the mechanical and other physical properties of a composite material or object. However, it has been found that carbon nanotubes typically tend to form a tangled mess resembling a hairball, which is difficult to work with. This and other difficulties have limited efforts toward realizing a composite material or object containing well-dispersed nanotubes with preferred orientations. It is to the provision of methods for producing composite materials or objects containing well-dispersed nanotubes with preferred orientations, and to composite materials or objects containing well-dispersed nanotube reinforcement with preferred orientations, that certain aspects of the invention are primarily directed.
Additionally, several new manufacturing processes, commonly referred to as solid freeform fabrication (SFF) or layer manufacturing (LM), have recently emerged to build parts point-by-point and layer-by-layer. These processes were developed for making models, material-processing toolings (e.g., molds and dies), and prototype parts. They are capable of producing 3-D solid objects directly from a computer-created model without part-specific tooling or human intervention. A SFF process also has potential as a cost-effective production process if the number of parts needed at a given time is relatively small. Use of SFF may reduce tool-making time and cost, and provide the opportunity to modify tool design without incurring high costs and lengthy time delays. A SFF process can be used to fabricate certain types of parts with a complex geometry which otherwise could not be made by traditional fabrication techniques such as machining, extrusion and injection molding.
Examples of SFF techniques are stereo lithography (SLa), selective laser sintering (SLS), 3-D printing, inkjet printing, laminated object manufacturing (LOM), fused deposition modeling (FDM), etc. SFF technology may be divided into three general levels of sophistication: The first is the ability to generate models or prototypes that clearly show the part design concept in three dimensions. All or most SFF techniques developed so far are capable of creating such models. The second level is the ability to produce parts that have acceptable dimensions and tolerances, and sufficient strength for preliminary evaluation in a simulated service environment. Although some progress has been made in attempting to achieve this ability, parts produced without fiber reinforcement, or with only short fibers, typically lack adequate structural integrity for many applications.
The third level is the ability to produce parts having high structural integrity and good dimensional tolerances, such that they can be placed in real operating systems. To date, little progress has been made toward fabricating SFF parts with this high level of structural integrity. Some preliminary attempts have been made to use stereo lithography-based techniques to fabricate both short and continuous fiber reinforced, UV-curable resin composites. In most cases, only composites with excessively low volume fractions of fibers are obtained using known fabrication methods and, hence, the resulting composites have exhibited low strength and stiffness, insufficient for many applications. Furthermore, such stereo lithography-based techniques typically allow use of only a laser-curable or UV-curable resin as the matrix material for a composite.
Fiber reinforced composites are known to have great stiffness, strength, damage tolerance, fatigue resistance, and corrosion resistance. However, currently available SFF technologies, in their present forms, typically do not lend themselves to the production of continuous fiber composite parts. The present invention, however, recognizes that selected SFF approaches (such as fused deposition modeling) can be modified and integrated with textile structure forming operations (such as selected fiber-laying steps in braiding, weaving and knitting) to produce parts on an essentially layer-by-layer basis. The parts produced by such a combination of SFF and textile operations, being of continuous fiber reinforced composite, are of superior structural integrity. The new processes of the present invention thus represent a major step forward toward achieving the highest level of sophistication in SFF.
The SFF techniques that potentially can be used to fabricate short fiber or particulate reinforced composite parts include fused deposition modeling (FDM), laminated object manufacturing (LOM) or related lamination-based process, and powder-dispensing techniques. As presently understood, the FDM process (e.g., U.S. Pat. No. 5,121,329; 1992 to S. S. Crump, incorporated herein by reference) operates by employing a heated nozzle to melt and extrude out a material such as nylon, ABS plastic (acrylonitrile butadiene-styrene) and wax in the form of a rod or filament The filament or rod is introduced into a channel of a nozzle inside which the rod/filament is driven by a motor and associated rollers to move like a piston. The front end, near a nozzle tip, of this piston is heated to become melted; the rear end or solid portion of this piston pushes the melted portion forward to exit through the nozzle tip. The nozzle is translated under the control of a computer system in accordance with previously sliced CAD data to trace out a 3-D object point by point and layer by layer. In principle, the filament may be composed of a fiber or particulate reinforcement dispersed in a matrix (e.g., a thermoplastic such as nylon). In this case, the resulting object would be a short fiber composite or particulate composite. The FDM method has been hitherto limited to low melting materials such as thermoplastics and wax and has not been practiced for preparation of metallic parts, possibly due to the difficulty in incorporating a high temperature nozzle in the FDM system.
Modified laminated object manufacturing (LOM) has been used to prepare polymer matrix and ceramic matrix composites (D. Klosterman, et al, in Proceedings of The 7th International Conference on Rapid Prototyping, Mar, 31, 1997 -Apr. 3, 1997, San Francisco, Calif., U.S.A., ed. By R. P. Chartoff, et al.; pp.43-50 and pp.283-292, incorporated herein by reference). As presently understood, the process involves, for instance, feeding, laminating and cutting thin sheets of prepregs (preimpregnated fiber preform) in a layer-by-layer fashion according to computer-sliced layer data representing cross sectional layers of a 3-D object. The process cycle typically consists of laminating a single sheet of prepreg to an existing stack, laser cutting the perimeter of the part cross section, and laser-dicing or “cubing” the waste material. After all layers have been completed, the part block is removed from the platform, and the excess material is removed to reveal the 3-D object This process results in large quantities of expensive prepreg materials being wasted.
In U.S. Pat. No. 5,514,232, issued May 7, 1996, incorporated herein by reference, Burns discloses a method and apparatus for automatic fabrication of a 3-D object from individual layers of fabrication material having a predetermined configuration. As presently understood, each layer of fabrication material is first deposited on a carrier substrate in a deposition station. The fabrication material along with the substrate are then transferred to a stacker station. At this stacker station the individual layers are stacked together, with successive layers being affixed to each other and the substrate being removed after affixation. One advantage of this method is that the deposition station may permit deposition of layers with variable colors or material compositions. In real practice, however, transferring a delicate, not fully consolidated layer from one station to another would typically tend to shift the layer position and distort the layer shape. The removal of individual layers from their substrate also tends to inflict changes in layer shape and position with respect to a previous layer, typically leading to inaccuracy in the resulting part.
In U.S. Pat. No. 5,301,863 issued on Apr. 12, 1994, incorporated herein by reference, Prinz and Weiss disclose a Shape Deposition Manufacturing (SDM) system. As presently understood, the system contains a material deposition station and a plurality of processing stations (for mask making, heat treating, packaging, complementary material deposition, shot peening, cleaning, shaping, sand-blasting, and inspection). Each processing station performs a separate function such that when the functions are performed in series, a layer of an object is produced and is prepared for the deposition of the next layer. This system requires an article transfer apparatus, a robot arm, to repetitively move the object supporting platform and any layers formed thereon out of the deposition station into one or more of the processing stations before returning to the deposition station for building the next layer. These additional operations in the processing stations tend to shift the relative position of the object with respect to the object platform. Further, the transfer apparatus may not precisely bring the object to its exact previous position. Hence, the subsequent layer may be deposited on an incorrect spot, thereby compromising part accuracy. The more processing stations that the growing object has to go through, the higher the chances are for the part accuracy to be lost. Such a complex and complicated process typically makes the over-all fabrication equipment bulky, heavy, expensive, and difficult to maintain. The equipment typically also requires attended operation, adding to expense.
In the composite manufacturing industry, numerous conventional methods are being practiced to produce continuous fiber reinforced composites. All of these methods are believed to have disadvantages or shortcomings. For example, the hand lay-up process is labor-intensive and the quality of the resulting composite part depends highly upon the skills of an operator. The combined process of prepreg preparation, cutting, lay-up, vacuum bagging, and autoclave or press curing is notoriously tedious, lengthy, and energy-intensive. In resin transfer molding (RTM), a dry reinforcement material, originally in the forms of roving, mat, fabric, or a combination, is cut and shaped into a preform The preform is then pre-rigidized by using a small amount of fast-curing resin to hold its shape during the subsequent operations. The preform is then placed in a mold, the mold is closed and resin is then injected into it. Resin must flow through the small channels inside a normally tightly-configured preform, expelling the air in the mold cavity, impregnating the preform and wetting out the fibers. The RTM process suffers from several drawbacks. First, it typically requires execution of two separate processes: preform preparation and resin impregnation. Complete impregnation of a dense or large-sized preform by a viscous resin can be very difficult. Second, it typically requires utilization of a mold, which is normally quite costly. Third, RTM typically is not suitable for fabricating complex-shaped parts (e.g., part with a hollow cavity).
Processes such as filament winding and pultrusion can be highly automated. However, filament winding generally is essentially limited to fabrication of convex-shaped hollow structures such as pressure vessels. Pultrusion can produce a variety of reinforced solid, tubular, or structural profiles. Unfortunately, these structures are essentially limited to be of a constant cross-section. Both filament winding and protrusion typically are not well-suited to production of complex-shaped parts. Although fiber placement and robotic tape-laying techniques can overcome some of the shortcomings of filament winding, they typically require the utilization of expensive, large and heavy equipment. The fiber placing or tape laying head is typically of a complex configuration and, hence, is easily subject to malfunction. These two techniques typically require highly specialized control software that is not usable with any other material processing machine. An overview of various composite processing techniques is available in B. Z. Jang, “Advanced Polymer Composites: Principles and Applications,” ASM International, Materials Park, Ohio, December 1994, incorporated herein by reference.
In summary, currently available SFF technologies, generally speaking, do not lend themselves to the production of continuous fiber composite parts. In general, current composite processing techniques are not capable of producing parts of a complex geometry, or producing parts of a specified geometry directly from a computer-aided design. Accordingly, it has been found desirable to develop a process and apparatus that can be used to fabricate continuous fiber reinforced composite parts of high structural integrity and complex geometry. It is further desirable that the process also has the capability of producing a three-dimensional object automatically in response to the computer-aided design of the object. These objectives can be achieved to some extent by the composite layer manufacturing (CLM) method of U.S. Pat. No. 5,936,861, Aug. 10, 1999 to Jang, et al., incorporated herein by reference. The CLM method involves mixing a fiber tow with a solidifying matrix material to form a pre-impregnated tow or “towpreg” and depositing the towpreg point by point and layer by layer on an object-supporting base member.
The present invention provides significant improvements in many aspects over the method disclosed in U.S. Pat. No. 5,936,861. For example, in one aspect, the present invention provides an effective method of generating the tool path (deposition path) along which a pre-impregnated tow is dispensed and deposited. The method minimizes the requirements for halting the deposition operation to cut off the towpreg tentatively from a dispensing nozzle and then to re-start the deposition operation at a different location.